1. Field of the Invention
The present invention relates to a fluorosulfonyl group-containing polymer which is a precursor of a sulfonic acid group-containing polymer useful as an ion-exchange membrane (e.g. a membrane to be used for brine electrolysis or polymer electrolyte fuel cells) or an electrolyte membrane to be used for a catalyst layer of a fuel cell; and a new fluorosulfonyl group-containing monomer which can be a raw material of the polymer. Further, the present invention relates to a process for producing the fluorosulfonyl group-containing monomer and a new compound useful as an intermediate for production of the monomer. Furthermore, it relates to a sulfonic acid group-containing polymer obtainable from the above fluorosulfonyl group-containing polymer and an electrolyte material for polymer electrolyte fuel cells, which comprises the sulfonic acid group-containing polymer.
2. Discussion of Background
Heretofore, a copolymer of a fluorinated monomer of the following formula and tetrafluoroethylene, has been used as a membrane for brine electrolysis, a membrane of a polymer electrolyte fuel cell or its catalyst layer. In the following formula, Y is a fluorine atom or a trifluoromethyl group, n is an integer of from 1 to 12, m is an integer of from 0 to 3, k is 0 or 1, and m+k>0:CF2═CF—(OCF2CFY)m—Ok—(CF2)n—SO2F
Further, the fluorosulfonyl group (—SO2F) in the copolymer can be converted to a sulfonic acid group (—SO3H) by alkali hydrolysis, followed by acid treatment.
When used for a brine electrolysis cell as a membrane having a high ion-exchange capacity, such a sulfonic acid group-containing polymer (hereinafter referred to as the sulfonic acid polymer) is a polymer which can reduce the power for the electrolysis. When the sulfonic acid polymer is used for a fuel cell, the polymer can improve the power generation energy efficiency. Further, the sulfonic acid polymer is preferably a polymer having a higher ion-exchange capacity and a lower electric resistance.
However, if the proportion of the fluorosulfonyl group-containing monomer to be used for copolymerization, is increased for a purpose of increasing the ion-exchange capacity of the sulfonic acid polymer, there has been a problem such that the molecular weight of the copolymer becomes low. A membrane formed by the copolymer having low molecular weight is insufficient in mechanical strength and durability, and thus it is not practically useful. It has been proposed to use a fluorosulfonyl group-containing monomer having plural fluorosulfonyl groups as a means to increase the ion-exchange capacity of the sulfonic acid polymer without increasing the proportion of the fluorosulfonyl group-containing monomer (Patent Document 1).
Further, in order to obtain a sulfonic acid polymer having a high molecular weight, the fluorosulfonyl group-containing monomer is required to have high copolymerizability with other fluoromonomers such as tetrafluoroethylene, but the conventional fluorosulfonyl group-containing monomer did not sufficiently have such copolymerizability. As a fluorosulfonyl group-containing monomer having a high polymerization reactivity, a perfluoro(2-methylene-1,3-dioxolane) derivative having a fluorosulfonyl group is known (Patent Documents 2, 3 and 4). However, such a derivative having plural fluorosulfonyl groups is not known.
Patent Document 1: WO2007/013532
Patent Document 2: WO03/037885
Patent Document 3: JP-A-2005-314388
Patent Document 4: JP-A-2006-290864